1. Field of the Invention
The present invention relates to a heat-sensitive positive-working lithographic printing plate precursor.
2. Description of the Related Art
Lithographic printing typically involves the use of a so-called printing master such as a printing plate which is mounted on a cylinder of a rotary printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to the image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e., ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e., water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called a plate precursor. A typical positive-working plate precursor includes a hydrophilic support and an oleophilic coating which is not readily soluble in an aqueous alkaline developer in the non-exposed state and becomes soluble in the developer after exposure to radiation. In addition to the well known photosensitive imaging materials which are suitable for UV contact exposure through a film mask (the so-called pre-sensitized plates), heat-sensitive printing plate precursors have also become very popular. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method (CtP) wherein the plate precursor is directly exposed, i.e., without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer or by particle coagulation of a thermoplastic polymer latex, and solubilization by the destruction of intermolecular interactions or by increasing the penetrability of a development barrier layer.
Although some of these thermal processes enable plate making without wet processing, the most popular thermal plates form an image by a heat-induced solubility difference in an alkaline developer between exposed and non-exposed areas of the coating. The coating typically includes an oleophilic binder of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working) by the image-wise exposure.
Typically, the oleophilic resin in a heat-sensitive plate is a phenolic resin such as novolac, resol or a polyvinylphenolic resin. The phenolic resin can be chemically modified whereby the phenolic monomeric unit is substituted by a group such as described in WO 99/01795, EP 934 822, EP 1 072 432, U.S. Pat. No. 3,929,488, EP 2 102 443, EP 2 102 444, EP 2 102 445, EP 2 102 446. The phenolic resin can also been mixed with other polymers as described in WO 2004/020484, U.S. Pat. No. 6,143,464, WO 2001/09682, EP 933 682, WO 99/63407, WO 2002/53626, EP 1 433 594 and EP 1 439 058. The coating can also be composed of two or more layers, each of them including one or more of the above described resins as described in, e.g., EP 864420, EP 909657, EP-A 1011970, EP-A 1263590, EP-A 1268660, EP-A 1072432, EP-A 1120246, EP-A 1303399, EP-A 1311394, EP-A 1211065, EP-A 1368413, EP-A 1241003, EP-A 1299238, EP-A 1262318, EP-A 1275498, EP-A 1291172, WO 2003/74287, WO 2004/33206, EP-A 1433594 and EP-A 1439058.
The binder described in EP 864420 and EP 909 657 is a copolymer which contains not less than 10 mol % of a monomer having a sulphonamide group wherein at least one hydrogen atom is linked to a nitrogen atom.